A metal matrix composite (MMC) is a composite material made of at least two constituent materials, one of which is metal. The other material(s) in an MMC can be another metal, ceramic material, organic material, or the like. Conventional MMCs include a metal matrix reinforced with one or more other materials in the form of particulates, whiskers, or fibers.
The reinforcement material is embedded into the metal matrix. The reinforcement material can be used to structurally reinforce the material or to change its physical, chemical, or mechanical properties such as wear resistance, friction coefficient, or thermal conductivity. The reinforcement material can be coated by such means as physical vapor deposition (PVD), chemical vapor deposition (CVD) or other suitable method to prevent undesirable chemical reactions with the metal matrix.
The reinforcement material can be characterized as being continuous or discontinuous depending on the aspect ratio of the material. For example, reinforcement materials having an aspect ratio greater than approximately 50:1 can be considered continuous while those having an aspect ratio less than or equal to approximately 50:1 can be considered discontinuous. Examples of continuous reinforcement materials include long fibers or whiskers and monofilament wires. Examples of discontinuous reinforcement materials include short fibers or whiskers and particles.
MMCs with continuous reinforcement are generally anisotropic meaning that their mechanical and/or physical properties vary depending on the direction of the material that is tested. This is usually due to the reinforcement material being embedded in the metal matrix in a certain direction. On other hand, MMCs with discontinuous reinforcement are generally isotropic meaning that their mechanical and/or physical properties are uniform in all directions and independent of the direction of the material that is tested. This is usually due to the reinforcement material being embedded homogenously in all directions.
Unfortunately, conventional MMCs and other composites suffer from a number of problems. One problem is that they are not particularly suited for applications where fracture mechanics are important. MMCs tend to fragment unpredictably in these situations. MMCs produce complex internal crack patters that are difficult to predict using standard fracture mechanics tests and analytical methods for metals, which assume self-similar crack extension—i.e., a crack will simply lengthen without changing shape.
Another problem is that the uses of conventional MMCs are limited by their structures and the materials used to make them. Applicant has discovered new applications for composite materials that include an internal skeleton structure.